1、外文资料GearsGears are vital factors in machinery，which are uses to transmit power or motion from one shaft to anotherThey may be used only to transmit motion from one part of a machine to another，or they may be used to change the speed or the torque of one shaft with relation to anotherOne of the first
2、 mechanism invented using gears wad the clockIn fact，a clock is little more than a train of gearsConsiderable study and research have been made on gears in recent years because of their wide use under exacting conditionsThey have to transmit heavier loads and run at high speeds than ever beforeThe e
3、ngineers and the machinists all consider gearing the prime element in nearly all classes of machinerySuper Gears Spur gears will be considered first for several reasonsIn the first place，they are simplest and the least expensive of gears and they may be used to transmit power between parallel shafts
4、，also，spur gears definitions are usually applicable to other types It is important go understand the following definitions，since they are important factors in the design of any equipment utilizing gearsDiametric Pitch The number of teeth per inch of pitch cirle diameter The diameter pitch is usually
5、 an integer A small number for the pitch implies a large tooth sizeMeshing spur gears must have the same diameter pitch The speed ratio is based on the fact that meshing gears may have different-sized pitch circles and hence different number of teethCircular Pitch The distance from a point on one to
6、oth to the corresponding point on an adjacent tooth，measrued along the pitch circleThis is a liner dimension and thus bas liner unitsPitch Circle The circle on which the ratio of the gear set is based，when two gears are meshing，the two pitch circles must be exactly tangent if the gears are to functi
7、on properlyThe tangency point is known as the pitch pointPressure Angle The angle between the line of action and a line perpendicular to the centerlines of the two gears in mesing Pressure Angles for spur gears are usually 145 or 20 degrees，although other values can be usedMeshing gears must have th
8、e same pressure anglesIn the case of a rack，the teeth have the straight sides inclined at an angle corresponding to the pressure angleBase CircleA circle tangent to the line of action （or pressure line ） The base circle is the imaginary circle about which an involutes cure is developed Most spur gea
9、rs follow an involutes cure from the base circle to the top of the tootch，this cure can be visualized by observing a point on a taut cord an it is unwound from a cylinder In a gear，the cylinder is the best circleAddendumThe radial distance form the pitch circle to the top of the tooth DedendumThe ra
10、dial distance from file pitch circle to the root of the toothClearanceThe difference between the addendum and the addendumFace WidthThe width of the tooth measured axiallyFace The surface between the pitch circle and the top of the toothFlank The surface between the pitch circle and the bottom of th
11、e toothHelical Gears These gears have their tooth element at an angle or helix to the axis of the gearThey are more difficult and expensive to make than spur gears，but are quieter and stronger They may be used to transmit power between parallel shafts at an angle to each in the same or different pla
12、nesHerringbone GearsA herringbone gear is equivalent to a right-hand and a left-hand helical gear placed side by sideBecause of the angle of the tooth，helical gears create considerable side thrust on the shaft A herringbone gear corrects this thrust by neutralizing it，allowing the use of a small thr
13、ust bearing instead of a large one and perhaps eliminating one altogetherOften a central groove is made round the gear for ease in machiningBevel Gears Bevel gears are used to connect shafts， which are not parallel to each otherUsually the shafts are 90 degTo each other， but they may be more or less
14、 than 90 degThe two meshing gears may have the same number of teeth for the purpose of changing direction of motion only，or they may have a different number of teeth for the purpose of changing both speed and direction The faces of the teeth lie on the surface of the frustum of a cone，therefore the
15、teeth elements are not parallel to each other it can be seen that this lack of parallelism creates a machining problem so that two passes with a tool must be madeThe tooth elements may be straight or spiral，so that we have plain anti spiral evel gearsWorm and Worm GearsA worm-and-worm-gear combinati
16、on is used chiefly where it is desired to obtain a high gear reduction in a limited space，normally the worm drivers the worm gear and is not reversible，that is to say，the worm gear can not drive the wormMost worms can be rotated in either direction，clockwise or counterclockwiseRacks A rack is a gear with an infinite radius，or a gear with its perimeter stretched out into a
